Classical quantum chemical solution of the FeMo-cofactor model to chemical accuracy and its implications.

We use classical quantum chemistry methods to estimate the ground-state energy to chemical accuracy in a model of the FeMo cofactor of nitrogenase, which is widely studied as a target of quantum computing. Our result relies on the insight that the ground-state problem can be characterized as one of ranking many competing, but largely single-configuration, states. This allows a combination of systematic high-order coupled cluster and density matrix renormalization group calculations together with an extrapolation protocol to obtain an accurate energy. Within the model, we identify several spin isomer candidates for the ground state that are degenerate to chemical accuracy. Beyond this model, we characterize the impact of additional electronic excitations and the cluster and protein geometric fluctuations on the low-lying electronic landscape. We find that many features of the landscape are retained in more detailed representations of nitrogenase, which points to the complexity of spectroscopic interpretations of the electronic structure of the FeMo cofactor.